Light trap

ABSTRACT

A light trap into which a beam of light is directed and caused to be reflected many times through a focal point over a different path on each reflection to achieve an increase in light energy density at the focal point from a given light energy beam. Very large gains may be achieved utilizing a concave ellipsoidal light reflector which reflects light directed from a primary focal point toward a secondary focal point along a common axis and a second light reflector which reflects light directed toward the secondary focal point back toward the primary focal point and the concave ellipsoidal reflector, the second light reflector being either an ellipsoidal or planar reflector, in combination with an appropriately oriented spherical reflector.

United States Patent r191 Hartley et al.

[ LIGHT TRAP [75] Inventors: Danny L. Hartley, Livermore, Calif;

Ronald A. Hill, Albuquerque, N. Mex.

[73] Assignee: The United States of America as represented by the United States Atomic Energy Commission, Washington, DC.

22 Filed: Feb. 14, 1973 21 Appl. No.: 332,541

[52] US. Cl 350/294, 350/299, 356/75 [5 1 1 t C "1::3':;:;.:' '.L;;':;r:.: [58] Field of Search 350/293, 294, 299, 7;

[451 July 23, 1974 3,748,014 7/1973 Beiser 350]? Primary Examiner-Ronald L. Wibert Assistant ExaminerMichael .l. Tokar Attorney, Agent, or Firm-John A. Horan; Dudley W. King; Richard E. Constant [57] ABSTRACT A light trap into which a beam of light is directed and caused to be reflected many times through a focal point over a different path on each reflection to achieve an increase in light energy density at the focal point from a given light energy beam. Very large gains may be achieved utilizing a concave ellipsoidal light reflector which reflects light directed from a primary focal point toward a secondary focal point along a common axis and-a second light reflector which reflects light directed toward the secondary focal point back toward the primary focal point and the concave ellipsoidal reflector, the second light reflector being either an ellipsoidal or planar reflector, in combination with an appropriately oriented spherical reflector.

5 Claims, 7 Drawing Figures LIGHT SPECTROMETER 24 GAIN SHEET 1 BF 2 LIGHT FlG. SPECTROMETER I I :1 x I I I0 Q 22!:

FIG 2b R=o.95 o I O l I 0 2 l 03 2C EdCENTRICITY FIG. 2a

o 0 l l o I l l I 7 o 2 3 4 5 0 an 0.2 0.3 0.4 0.5

ELLIPSOIDAL REFLECTOR RADIUS (CENTIMETERS) ECCENTRICITY P JUL 31914 3.825.325 SHEET 2 BF 2 FIG. 3

FIG.

BACKGROUND OF INVENTION There are some applications of light where it is desirable to provide very high energy light densities at some location or region for some desired purpose. High energy density light may be produced from various light sources, such as from lasers, mercury arc lamps, or the like, and these energies increased by various light source operating techniques like Q-switching and other mechanisms appropriate to the particular light source. It is readily apparent that as the energy requirements of the light beams are increased, the light sources require ever increasing amounts of power or energy to produce these light beam energies.

In many of these applications, only a small percentage of the light energy in a beam of light may be utilized. Thus, even though the intensity or energy of a light beam may be increased by appropriate means, as referred to above, the efficiency of the device remains low. Some attempts have been made to provide multiple reflections of a light beam so as to cause the light beam to traverse certain locations more than once to achieve higher percentage of utilization of light energy in a given beam. However, these prior devices have generally been limited as to the number of reflections that may be achieved or the reflection paths have been directed over a relatively wide area. These prior multiple path light beam deflectors, for example, may utilize a pair of planar mirrors at some angle with respect to each other or combinations of concave or spherical mirrors or portions of spherical mirrors and planar mirrors. In most of these prior systems, the light beam paths generally neither crossed nor followed or approached the same path or location more than twice.

Ellipsoidal mirrors have also been used in the past for directing light from a first focus of themirror to a second focus with a change in f-number or to collect light from a point source located at one of the foci and then imaging it at the other focus, again with a change in fnumber. These prior ellipsoidal mirror devices generally utilize only a single reflection from the mirror surface.

As noted above, high energy density light beams are useful in various applications. For example, a light beam may be directed through a gas sample and the Raman scattered light sensed to identify the molecular species of the gas, measure the concentration of the gas, study the molecular structure of the gas, and the like. The total intensity of molecular scattering from the gas may typically be of the order of of the incident light beam intensity, of which about one percent may be attributed to the Raman spectrum. It can be seen that even with high powered light sources, Raman scattering can still be at extremely low intensities. Since the Raman scattering is of such extremely low level, Raman signals are often detected by highly sensitive photon counting equipment which, for various reasons, are readily accessible to error. Also, because of the nature of the measurements being made, the Raman scattering is generally sensed from a relatively small area or volume to insure accuracy of measurements. Attempts have been made to increase light energy density in such applications using a concave mirror with either another concave mirror or a planar mirror. These attempts have 2 been at least partially successful, but it would be desirable to produce even larger light fluxes or light energy densities in a given volume from a unit light source.

SUMMARY OF INVENTION In view of the above, it is an object of this invention to provide an optical device which effectively traps a beam of light within a volume and reflects the light back and forth repetitively through the volume near a focal point.

It is a further object of this invention to provide such a light trapping function within a relatively simple optical reflector system which effectively increases the light intensity at a given location by a factor of about 4 over the previously achieved.

Various other objects and advantages will appear from the following description of the invention, and the most novel features will be particularly pointed out hereinafter in connection with the appended claims. It will be understood that various changes in the details and arrangements of the parts, which are herein described and illustrated in order to explain the natureof the invention, may be made by those skilled in the art.

The invention comprises an ellipsoidal reflector optically coupled to either a planar or second ellipsoidal reflector adapted to trap a beam of light in an optical cavity formed by said reflectors by cycling thereof back and forth between said reflectors, together with a portion of spherical reflector disposed annularly about one of said reflectors with-center of curvature to reflect magnified images back into the optical cavity through 3 additional complete cycles.

DESCRIPTION OF DRAWING The present invention is illustrated in the accompanying drawing wherein:

FIG. 1 is a cross-sectional and somewhat diagrammatic view of a light trap which incorporates the increased gain features of this invention and its use with a Raman spectroscope;

FIG. 2a is a graph of beam passes (n), initial light beam image size (d and maximum light beam image size (D) with increasing eccentricities for the light trap of FIG. 1;

FIG. 2b is a graph of scattered light gain for the light trap of FIG. 1 as a function of reflectivity and eccentricity of the reflectors;

FIG. 20 is a graph of gain as a function of the radius of the ellipsoidal reflector of FIG. 1 for different eccentricities;

FIG. 3 is a diagrammatic view of another arrangement of a light trap;

FIG. 4 is a diagrammatic view of still another optical light trap having increased gain capabilities in accordance with this invention; and

FIG. 5 is another diagrammatic view of an optical device which utilizes the light trapping capabilities of this invention.

DETAILED DESCRIPTION For purposes of this invention, the term ellipsoidal reflector is defined as a symmetrical section or porin this invention, a light trap consisting of a concave and preferably ellipsoidal reflector which reflects light directed from a first focal point toward a second focal point further removed from the concave reflector and another light reflector which reflects light directed towards the second focal point back towards the first focal point in the concave reflector is provided with a spherical reflector having a center of curvature at one of said focal points and positioned behind and annularly emcompassing one of said reflectors. As the light is reflected back and forth between the concave reflector and the other reflector, the center of the light beam approaches parallelism and coincidence with the axis joining the focal points, which axis is generally the major axis of the ellipsoidal reflector. The orientation of the reflectors is such that the light beam may be simultaneously magnified during certain of the traversals of the light trap and at some point escape around the periphery of either the concave or other light reflector ,as an annular fringe of light and be caught by the spherical reflector and returned toward one of the focal points. The reflectors are arranged so as to provide four complete cycles of light beam reflections.

In the arrangement of the light trap shown in FIG. 1, this light trapping may be achieved by the concave ellipsoidal mirroror reflector 10, which has a first or near focal point 12 and a second or far focal point 14 situated along its major axis, a planar mirror or reflector 16 and an annular spherical mirror or reflector 18. Each of the reflectors may be made of any appropriate material with'reflective surfaces of very high reflectivity in desired wavelength ranges such as a glass with suitable dielectric coatings toprovide reflection at the surface of the glass. The reflective surface of reflector 16 is preferably positioned at the midway point between focal points 12 and 14 and perpendicular to the major axis of reflector 10. Annular spherical reflector 18 may be made a .unitaryand composite part of planar reflector 16 about the periphery thereof and preferably has a radius of curvature centered at focal point 12 and equal to the distance between focal point 12 and the periphery of the reflecting surface of reflector 16. The combination of concave reflector l0, planar reflector 16 and annular spherical reflector 18 forms an optical cavity 20.

Any light beams injected into the optical cavity 20 formed by reflectors 10, 16 and 18 with an appropriate initial direction, may be trapped within the optical .cav-

ity for some number of reflections. For example, a light beam'22 may be directed from an appropriate light source 24 through an optical aperture or opening 26 in annular spherical reflector 18 toward focal point 12 and reflector 10, as shown. Light source 24 may be any appropriate light producing apparatus with or without appropriate lenses, mirrors or the like to produce a desired size and intensity of light beam. Particularly appropriate light sources may be continuous wave or pulsed laser systems which produce monochromatic beams of light. The initial light beam may also be directed, if desired, from the general direction of concave reflector toward focal point l4,and against planar reflector 16.

The light beam 22, after passing through focal point 12, will reflect from the reflective surface of concave reflector 10 toward focal point 14 to the reflective surface of planar reflector 16, as shown by the reflected beam 22a. Beam 22a will in turn be reflected from reflector 16 back towards focal point 12 and reflector 10, as shown by light beam 22b. The light beam will continue to be reflected from the respective surfaces toward the respective focal points, succeeding beam paths continuously approaching the major axis and passing through focal point 12 after every second reflection.

Planar mirror 16 is dimensioned so that the diameter of its reflective surface at its location halfway between focal points 12 and 14 is no greater than an imaginary cone having an apex at focal point 14 and a base about the periphery of concave reflector 10. Whenthe light beam is reflected through focal point 12, the diameter of the beam is magnified in accordance with the formulas e c/a where;

M magnification;

e eccentricity of concave reflector 10;

c half of the separation distance between the focal points; and

a distance between the concave reflective surface along its major axis to a point midway between the focal points.

The spot diameters of the successive return beams in the focal planes are demagnifled and converged to the major axis. When the magnified image diameter, or a portion thereof, is larger than the imaginary cone referred to above, the light beam may exceed the diameter of planar reflector 16 and attempt .to escape as an annular fringe about its periphery. The light rays in the portion of this beam intercepted by reflector 16 may make an additional pass through the optical cavity 20 before another magnified portion exceeds the diameter of planar reflector 16. The number of passes (it) that a light beam may take through optical cavity- 20 before any light escapes around the periphery of planar reflector 16 is a function of the magnification (M),'the initial image size (d of light beam 22 at focal point 12 and the diameter (D) at the focal point 12 of the cone formed by the periphery of concave mirror 10 and its apex at focal point 14. This may be shown by the following formula:

n integer [ln(D/d0)/ln M] Since the planar reflector 16 is circumscribed by annular spherical mirror 18 whose origin is coincident with focal point 12, the escaping light beam or light beam portion about planar reflector 16 will be directed back into optical cavity 20 along an inverted path. Because the redirected light follows an inverted path, the light beam will make as many passes through the optical cavity as in the first set of passes and eventually arrive at the reflective surface of annular spherical reflector 18 on the side of the planar mirror 16 diagonally opposite the entrance passageway 26. It should be noted that for each pass of light through focal point 12 that a light beam return pass is also made through the focal region about focal point 12 so that a complete cycleof light beam reflections and traversals of optical cavity 20 before escape of light around the periphery of planar reflector 16 requires 2n light beam passes through the focal region around focal point 12. Thus, a light beam portion that made 2n 2 passes through 5 the focal region before arriving at the spherical reflector 26 makes a total of 4n 4 passes before returning to the sphere at a spot opposite the entrance passageway 26. A light beam that made 211 +4 passes through the focal region before arriving at the spherical reflector 18 makes a total of 4n 8 passes before returning to the spherical reflector at the same spot opposite the entrance passageway 26. Each successive outer cone of light that is redirected back into the cell by the spherical reflector, eventually arrives back at the spherical reflector at a point diametrically across the planar reflector 16 from the entrance passageway 26. These light beams are returned in the same manner to the origin of the spherical reflector, e.g. near focal point 12 of concave reflector 10, where the beams go through a third set of passes and another cycle, inverted with respect to but otherwise identical in nature to the first cycle or set of passes. As in the first set of passes, the light beams are magnified again and eventually try to escape around the periphery of the reflector 16 towards the spherical reflector 18 and are thence returned for a fourth set of passes, inverted with respect to but otherwise identical to the second set of passes. The light beams of the fourth cycle eventually return to the spherical reflector 18 at the entrance passageway 26 where they escape from the optical cavity 20.

It is generally desirable to maintain the magnification, and thus the eccentricity of concave reflector 10, to as low a level as may be practical to maintain light energy densities at desired levels for the longest period of time and to provide the greatest number of light beam passes as possible. It has been found that the maximum eccentricity should be less than about 0.2 to keep the efficiency of the light trap at reasonable levels. The maximum image size (D) which may be enclosed within the light trap of FIG. 1, before the light beam begins to impinge against spherical reflector 18 may be determined from the following formulas:

D 4 Tan 0:

where;

r radius of ellipsoidal reflector; and a angle between major axis and line drawn from periphery of ellipsoidal reflector to far focal point. From these formulas, a light trap may be designed to produce some desired number of reflections in each cycle from an initial desired image size and other appropriate combination of initial parameters. The total intensity or energy density (I) of light at the focal point 12 and around the focal region after four complete cycles through the focus with a mirror reflectivity (R) and an initial light beam intensity (I may be determined from the formula:

The total light intensity gain at the focal region around focal point 12 may be determined by dividing I by I It can be seen that the reflectivity should be maintained to as high a level as possible, such as generally above 99 percent, while the mangification is maintained generally to the lowest levels achievable.

Various uses, as mentioned above, may be made of the light beam which is thus trapped in optical cavity 20. For example, a gas may be located within or passed through the optical cavity 20 and a portion of the light scattered from the gas in the focal region near focal point 12 may be sensed and measured through an appropriate viewing slit and optical light scattering collection apparatus bya light spectrometer 28. The scattered light is shown diagrammatically by beam 30. Light spectrometer 28 may include appropriate photosensitive or photomultiplier detectors, or the like, coupled to suitable recorders which will measure and indicate the amplitude and wavelength of the scattered light. Since the light beam 22, which has an energy level determined by the light source 24 and optical losses produced therein and by any other optical elements, is passed repetitively through or near focal point 12, any gas at focal point 12 will be subjected to the light energy at each pass and produce scattered light for sensing by light spectrometer 28. The amplitude of the scattered light is thus enhanced by the light trap by an amount approximating the number of passes of the light beam between the reflective surfaces of reflectors 10, 16 and 18 and the energy density of this light beam at each pass through the focal region near focal point 12 and the viewing slit or area. Other utilization means may also be appropriately coupled to the light trap in a similar manner. It has been found that the viewing slit should be of height equal to the maximum image size (D) to provide the maximum light beam intensity gain. The total scattered light intensity (I) in terms of the intensity (I,,) scattered in one light beam pass may be found from the same general formula (6) above.

Plots of the pertinent parameters specifying the gain of the light trap are shown in FIGS. 2a, 2b and 20.

These curves represent an arrangement with a 15.24

centimeters, r 3.81 centimeters and a light wave-- length of 5000 angstroms. The relationship of n, d (in terms of microns) and D (in terms of millimeters) with changing eccentricities is shown in FIG. 2a. It should be noted that as the eccentricity approaches zero, the image diameter (d,,) approaches the maximum magnified image diameter (D) and d may become equal to D at an eccentricity of about 0.004.Thus, as the eccentricity approaches zero, n goes through a maximum and then goes to about zero. A plot of [/1 is illustrated in FIG. 2b for a range of values of the eccentricity and reflectivity for the lighttrap of FIG. 1. With a reflectivity of about 99 percent a maximum gain of about 100 may be obtained with an eccentricity of about 0.009 while for a reflectivity of about 99.9 percent, a maximum gain of about 612 may be obtained with an eccentricity of about 0.0085. The dependence of gain on the radius of the concave reflector 10 for various values of eccentricity and for a reflectivity of about 99.9 percent is shown in FIG. 20. It can be seen that a larger diameter concave reflector 10 has an advantage with eccentricities less than about 0.15.

A light trap having the dimensions noted above, and with an eccentricity of-0.2 in the arrangement of FIG. 1 with a 1 watt argon-ion laser operating at about 4880 angstroms, exhibited a measured gain of about 92 with a Raman line at 5506 angstroms compared to the Raman signal with only one beam which may be roughly equivalent to the scattered light from a 92 watt laser with one beam.

As noted above, the gains may be maximized with various small values of eccentricity of the concave reflector 10, for example with an eccentricity of about 0.01. In practice however, such small eccentricities may be difficult to build and use as the planar reflector 16 may have a very small diameter (for example about 0.75 millimeters) and the scattering region would be very close to the planar reflector (within about 1.5 millimeters). This may be partially circumvented by using two concave reflectors having coincident focal points with an appropriate spherical reflector position behind each of the concave reflectors to redirect magnified escaping light back into the optical cavity. Even though the f-number of such an optical cavity would be quite large, in the neighborhood of about 250,'such may be operated with a laser light source. It is important that the surfaces of the reflectors, whatever arrangement is used, be kept at maximum cleanliness and at maximum reflectivity, preferably approaching 99.9 percent, to

obtain and maintain high gains. I

This same operation may be achieved using the embodiment of FIG. 3 in which the planar mirror 16 is selected also to have a diameter around which the trapped light beam may escape. The escaping light may be .caught and returned to the optical cavity by a spherical mirror 40 shown as an annular reflector. disposed symmetrically encompassing mirror 16 and having a center of curvature at focal point 12, similar to annular spherical portion 32 in FIG. 1. A light beam 42 may be entered into the optical cavity 18 through a passageway 44 in spherical reflector 40 in the same manner as in FIG. 1 and provide similar enhanced light trapping. However, a portion of the image reversed and reflected by the spherical reflector 40 may be obscured during each cycle by the planar reflector 16 positioned adjacent thereto, and thus be lost.

In the embodiment of FIG. 4, the planar reflector 46 may be at a diameter sufficiently large to cause the magnified light to escape around the ellipsoidal reflector from magnification in the manner described above. With a spherical reflector 48 (shown in annular configuration) positioned behind ellipsoidal reflector 10 with a diameter exceeding reflector 10 and with a center of curvature at far vocal point 14 (the curvature of reflector 48 is shown exaggerated for purpose of illustration and emphasis), an appropriate light beam trapped between the optical cavity formed by reflectors 10 and 46 may be caused to traverse the optical cavity four complete trapping cycles in the same manner as described above for FIG. 1. For example, the light beam 50 may be-directed through a passageway 52 in spherical reflector 48 towards focal point 14 and planar reflector 46. The light beam will betrapped in optical cavity 18 and after one cycle will attempt to escape around ellipsoidal reflector 10 against spherical reflector 48 and then be returned and inverted into optical cavity 18, etc. This arrangement may produce similar losses as the embodiment of FIG. 3 from partial masking of the inverted image which may be overcome by encompassing reflector '10 with an annular spherical mirror similar to spherical reflector 18 in FIG. 1.

Light beams may also be trapped in the optical cavity 54 of FIG. 5 in the same manner as described above utilizing an ellipsoidal reflector 56, a prism (not shown) or planar reflector 58 positioned at the near focal point 60 of reflector 56 at some appropriate angle to the major axis thereof and a planar reflector 62 and spherical reflector portion 64 appropriately positioned in optical alignment with reflectors 56 and 58. In the embodiment shown, reflector 58 is at an angle of 45 with the major axis and reflector 62 is parallel to the major axis of the ellipsoidal reflector 56. Reflector 62 is positioned a distance from focal point 60 equal to half of the separation distance between focal point 60 and far focal point 68. If it is desired, planar reflector 58 may be made partially reflective and any light passing therethrough appropriately detected by some suitable photosensitive detector so as to utilize the intensity enhancement characteristics of optical cavity 54 to increase the sensitivity of the photosensitive detector. Any appropriate light beam 66 may be injected into the optical cavity in a similar manner as described above, such as through opening 72 in reflector portion 64.

What is claimed is: 1. A light trap for preventing escape of annular fringe light comprising a concave ellipsoidal light reflector means for reflecting light directed from its near focal point toward its far focal point, said focal points being aligned along a common axis; a second light reflector means for reflecting light directed toward said far focal point back toward said near focal point and said concave ellipsoidal light reflector means; means for'directing a beam of light through one of the focal points of said concave ellipsoidal light reflector means toward one of said reflector means for producing a beam of trapped light reflected alternately from each of said reflector means in optical paths which successively approach said common axis and pass through said near focus, and an annular concave spherical curvature reflector means in addition to said concave ellipsoidal and said second reflector means generally encompassing one of said reflector means with inner diameter'portions no less than outer diameter of said encompassed reflector means and facing the other reflector means and having its center of curvature coincident with one of said focal points for reflecting annular fringe light and other light coming from said one focal point and escaping around said encompassed reflector means back toward said one focal point for trapping said escaping light between said concave ellipsoidal and second reflector means. Y

2. The light trap of claim 1 including an optical aperture through a portion of said spherical reflector means radially outward of said encompassed reflector means for conveyance of said light beam into said light trap through a focal point towards one of said other reflector means and for egress of the light beam after trapping thereof between said reflector means.

3. The light trap of claim 2 wherein said second reflector means includes a planar light reflector disposed perpendicular to said axis midway between said focal points and facing said concave ellipsoidal light reflector means and said concave spherical reflector means is disposed annularly about said planar reflector with its center of curvature coincident with said near focal point, said reflector and all of said reflector means having reflectivities of greater than about 99 percent and said concave ellipsoidal reflector means having an eccentricity of from about .01 to about .2.

4. The light trap of claim 1 wherein said second reflector means includes a first planar reflector disposed at an angle to said axis at said near focal point, and a planar reflector disposed laterally of said axis at an angle to said first planar reflector and aligned optically with said concave ellipsoidal light reflector means through said first planar reflector.

5. The light trap of claim 4 including means for detecting light directed against said first planar reflector. 

1. A light trap for preventing escape of annular fringe light comprising a concave ellipsoidal light reflector means for reflecting light directed from its near focal point toward its far focal point, said focal points being aligned along a common axis; a second light reflector means for reflecting light directed toward said far focal point back toward said near focal point and said concave ellipsoidal light reflector means; means for directing a beam of light through one of the focal points of said concave ellipsoidal light reflector means toward one of said reflector means for producing a beam of trapped light reflected alternately from each of said reflector means in optical paths which successively approach said common axis and pass through said near focus, and an annular concave spherical curvature reflector means in addition to said concave ellipsoidal and said second reflector means generally encompassing one of said reflector means with inner diameter portions no less than outer diameter of said encompassed reflector means and facing the other reflector means and having its center of curvature coincident with one of said focal points for reflecting annular fringe light and other light coming from said one focal point and escaping around said encompassed reflector means back toward said one focal point for trapping said escaping light between said concave ellipsoidal and second reflector means.
 2. The light trap of claim 1 including an optical aperture through a portion of said spherical reflector means radially outward of said encompassed reflector means for conveyance of said light beam into said light trap through a focal point towards one of said other reflector means and for egress of the light beam after trapping thereof between said reflector means.
 3. The light trap of claim 2 wherein said second reflector means includes a planar light reflector disposed perpendicular to said axis midway between said focal points and facing said concave ellipsoidal light reflector means and said concave spherical reflector means is disposed annularly about said planar reflector with its center of curvature coincident with said near focal point, said reflector and all of said reflector means having reflectivities of greater than about 99 percent and said concave ellipsoidal reflector means having an eccentricity of from about .01 to about .2.
 4. The light trap of claim 1 wherein said second reflector means includes a first planar reflector disposed at an angle to said axis at said near focal point, and a planar reflector disposed laterally of said axis at an angle to said first planar reflector and aligned optically with said concave ellipsoidal light reflector means through said first planar reflector.
 5. The light trap of claim 4 including means for detecting light directed against said first planar reflector. 